DESCRIPTION: In the first five years of this project, the principal investigator has completed baseline studies of 197 non-obese, preadolescent girls at high and low risk for the development of obesity as indicated by presence or absence of parental obesity. Data gathered includes the measurement of 24-hour energy expenditure using the doubly labeled water technique, resting metabolic rate, maximal oxygen consumption, body composition (total body water, anthropometry, BIA), as well as physical activity diaries and questionnaires, food diaries and questionnaires, and parental obesity status and activity questionnaires. It is now proposed that this cohort will be followed prospectively to determine whether girls who become obese were those with reduction in one or more components of energy expenditure at baseline. Longitudinal measurements of energy expenditure will be performed in a subcohort, and this information will be used to cross- validate other measures of physical activity. In addition, the investigators propose to measure abdominal fat using magnetic resonance imaging in girls at menarche and four years after menarche. These data will be combined with yearly measurements of plasma hormone concentrations, as well as lifestyle factors, to determine whether these are predictors of visceral fat gain.